Saints topped the table heading into the ultimate round but were made to sweat after a 17-16 defeat at Huddersfield Giants on Friday left their fate in the hands of Castleford Tigers.
However, Castleford were unable to crown a superb campaign by securing top spot and instead finished fourth courtesy of a 28-6 away loss to Catalan Dragons on Saturday.
Saints and Castleford will now meet in a qualifying play-off at Langtree Park on Friday, with the winner advancing to the semi-finals.
The other qualifying play-off will see Huddersfield travel to Wigan Warriors, who claimed second spot with a superb comeback victory over Warrington Wolves on Friday that ensures the latter have to settle for fifth in the table and an elimination play-off against Widnes Vikings on Saturday.
Catalan and sixth-placed Leeds Rhinos also face an elimination play-off at Headingley on the same day.
Wigan and Huddersfield are both likely to be buoyed by the manner of their successes this week.
The former trailed 18-4 to Warrington, for whom the prolific Joel Monaghan registered a brace, at half-time but came roaring back after the interval.
Joe Burgess added to his opening score and Josh Charnley went over for a second-half double before Blake Green's try tipped the contest decisively in the Warriors' favour.
Huddersfield prevailed against Saints thanks to a 71st-minute drop goal from Leroy Cudjoe.
Tommy Makinson, James Roby and Lance Hohaia scored tries for St Helens, who led 10-0 in the first half before Alex Walmsley was sent off for a dangerous tackle on Luke Robinson.
Cudjoe claimed his side's final try after Jake Connor and Shaun Lunt had also crossed.
Knowing victory would earn them top spot for the first time in their 88-year history, Castleford were well beaten by Catalan in Perpignan, Morgan Escare setting up a comfortable home win with two first-half tries.
Leeds are the only team to have won Super League from a fifth-place finish and will now seek to go one better.
The Rhinos' poor recent form continued with a 24-19 loss to Hull FC, who came from 15 points behind to round off an otherwise disappointing campaign in style.
Widnes will also go into their play-off tie on the back of a defeat, having been thumped 36-6 by Salford Red Devils.
Elsewhere, Omari Caro scored a hat-trick as Hull KR won 42-18 at Wakefield Trinity Wildcats and there was a high-scoring match between relegated duo London Broncos and Bradford Bulls, the latter leaving the capital with a 46-36 triumph.
Bradford's Luke Gale stared in a thrilling contest with four tries.